The subject matter of this application relates to amplifiers capable of being used in CATV transmission networks.
Linear RF amplifiers are often designed as class A amplifiers, which are one of the most commonly used amplifiers because of their excellent linearity. In a class A amplifier, the transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform, producing minimum distortion and maximum amplitude to the output since there is no crossover distortion to the output waveform, even during the negative half of the cycle. However, because the transistor is always biased “ON,” one of the main disadvantages of class A amplifiers is that their efficiency is very low as the constant and usually large currents cause a considerable amount of power to be lost.
This is particularly true in CATV transmission networks. When operated with a broadband signal containing many RF channels, the voltage bias point of the amplifier must typically be around 5 times the rms output voltage of the output stage when the probability of clipping must be in the ppm range. This is necessary in order to ensure that the amplifier can support peak output voltages. Such low clip probabilities are typical in CATV systems that support QAM channels with high complexity such as QAM256. The bias current of the amplifier similarly must be around 5 times the rms signal current in the amplifier output stage. As a result the power efficiency that can be attained is very low, on the order of 4% and often less.
A class B amplifier, conversely, has a pair of transistors that each conduct alternatingly only for one half cycle of the input signal. Since the active devices are switched off for half the input cycle, the active device dissipates less power and hence the efficiency is improved. However, since each active device truncates half the input signal and the output signal is the sum of the outputs of the active devices cross-over distortion that occurs when one device switches off and the other switches on is high in Class B amplifiers.
Class AB amplifiers, which are widely used in audio systems, use a bias current set at a non-zero value with a magnitude much lower than the peak output current, resulting in improved power efficiency relative to class A amplifiers. The output is configured with a transistor pair such that one transistor pulls positive voltages high and a second transistor can push negative voltages low. For small voltage magnitudes, both transistors are active but for large voltage magnitudes (either positive or negative) only one transistor is active whereas the current on the other output transistor can reach zero or is often held at a low minimum value. While some signal distortion is induced due to the transition in operation where either or both transistors are active, such distortions can be minimized by designing strong negative feedback into the amplifier; a fraction of the output signal is compared to the input signal and a correction signal is provided with high gain to the output signal if the output signal fraction deviates from the input signal. As a result, the output signal is held close to a multiple of the input signal and distortions are low.
In RF amplifiers in a push-pull configuration, typically an output transformer is used where both transistors can for instance be N-type FETs, and both can be driven with a signal set relative to ground, as opposed to some floating node. The output transformer has a differential input such that the difference of the transistor output is presented at the transformer output. Feedback in an RF amplifier is generally limited, due to the high signal bandwidth that needs to be amplified. Too much delay in the feedback path combined with a high gain for the correction signal will lead to amplifier oscillation due to the inevitable signal delay or phase shift in the feedback path. This complicates the implementation of a class AB amplifier, which produces distortions due to the transitions in the operating mode of the output transistors. Second, turning an RF transistor off can produce very high distortions when the transistor needs to be turned on again, it is preferable to prevent a complete turn-off under all conditions and instead ensure a minimum controlled current. Whereas ensuring a minimum current is not difficult when using a floating (output) node where the two transistors are connected it is less trivial in the RF amplifier where transistors are referenced to ground; use of the floating output node would be very difficult due to parasitic capacitance of that node to ground. This is very significant at the high output frequencies produced by an RF amplifier.
In RF amplifiers an alternate method to reduce distortion using feedback is feed-forward distortion compensation. When using feed-forward distortion compensation, the output of a first RF amplifier is provided to an RF coupler. A fraction of the output signal is coupled out and that fraction is compared to the input signal. The difference, due to distortion, is provided to a second, smaller amplifier with the same gain as the first amplifier and added to the output of the first amplifier such that the distortion in the combined signal is cancelled. Because the distortion power is typically much lower than signal power, the power handling capability of the second amplifier that only handles distortion may be much lower than that of the first RF amplifier. However, such a design suffers from complexity; it requires two extra couplers, an extra amplifiers and fine tuning of the gain and phase delay of the signal paths to ensure consistent distortion cancellation. Nevertheless results can be good such as 20 dB of distortion cancellation. A second disadvantage of the use of couplers is the inevitable loss of RF couplers; a fraction of the output power of the amplifier is dissipated in the output couplers.
What is desired, therefore, is an improved amplifier suitable for use in CATV networks such that high power efficiency can be achieved with linearity.